1. Field of the Invention
The present invention relates to multilayer structures used in extreme ultraviolet lithography, and more specifically, it relates to an interdiffusion resisting interlayer placed between the individual layers of the multilayer structure.
2. Description of Related Art
Success of extreme ultraviolet lithography (EUVL) is highly dependant on the throughput of the lithography tools. The throughput is an integral of the reflectance and of the intensity spectrum of the source over the wavelength region of interest. The reflectance is a product of reflectance curves from all reflective mirrors in the lithography tool. For example, in a lithography machine with nine reflective mirrors the reflectance under the integral would be R9. Therefore, any improvement in the reflectance leads to a substantial increase in the throughput of the lithography tool.
Mo/Si multilayers are deposited as a periodic stack of alternating Mo and Si layers. Although the reflectivity from each interface in the multilayer coating is small, the reflections from all the interfaces can interfere constructively to yield a total reflectivity of about 74% at wavelengths above Si L-edge at 12.5 nm. Mo and Si layers are separated by interlayers of a mixture of Mo and Si (amorphous molybdenum silicide) that considerably reduces the reflectivity of the coating. A practical multilayer, for example, has usually 10% (relative) lower reflectance than a multilayer with perfectly sharp interfaces. Thus, one issue is how to prevent the two materials (Mo and Si, in this case) from interdiffusing to form a compound and how to optimize the interface structure formed by inter diffusion.
U.S. Pat. No. 5,319,695, titled xe2x80x9cMultilayer Film Reflector For Soft X-Raysxe2x80x9d describes a soft X-ray multilayer film reflector which has a multilayer film structure formed adjacent higher and lower refractive index layers that are separated from one another by a hydrogenated interface layer that is thinner than either of the adjacent layers. The present invention separates the layers of a multilayer reflector without the use of the hydrogenated interface layer.
The other problem is that these multilayer coatings have to be thermally stable. This is especially important for multilayers used as reflective coatings on masks that undergo different etching procedures and thermal cycling. Currently, Mo/Si multilayers cannot be exposed to temperatures above 120 degrees Celsius even for a short time because the activation energy is so low that further diffusion and the growth of molybdenum silicide is inevitable. In order to keep the reflectance and the multilayer period thickness (which determines the wavelength at which these multilayer will reflect) stable one needs to find a way to increase the activation energy for diffusion of Si in Mo and Mo in Si.
It is an object of the present invention to control the interdiffusion of the two materials of an EUVL multilayer coating upon thermal processing to improve the thermal stability of Mo/Si interfaces and hence the multilayer.
It is also an object of this invention to optimize the interface structure thus optimizing performance.
These and other objects will be apparent based on the disclosure herein.
The invention is an EUVL multilayer structure formed with an interface layer placed between alternating layers of an absorber layer and a spacer layer. The interface layer comprises a material that controls interdiffusion between the absorber layer and the spacer layer. Materials usable as the interface layer include boron carbide and carbon with boron based compounds. These materials are characterized as having a low absorption of EUV and X-ray wavelengths. The present invention separates the layers of a multilayer reflector without the use of the hydrogenated interface layer.
For Mo/Si multilayers, the present inventors have discovered that the interface layer slightly reduces the formation of molybdenum silicide by the formation of a Mo, Si, B, C interfacial layer. For Mo/Si and Mo/Be multilayers, increases in the reflectance of EUV and X-ray wavelengths and increases in the thermal stability have been observed. A method for making an EUVL multilayer structure that includes the interface layer is provided as well.